I don't know how well anything will work with only 32mb of ram, but our company is using RAV antivirus. We bought it from MyEZserver (
http://www.myezserver.com/) which is owned by Darrel May (I think), who contributes to these very forums. We've been more than pleased with its performance, and it works wonderfully for viruses and spam filtering.
Darrel has made a server manager plug in for all the important settings in RAV, so it's very easy to set up and use. The price is also great (be sure to convert the Canadian dollars to US dollars when you're looking at the prices).
Like I said, I can't vouch for the performance in your system, because we're running it on a pretty healthy server hardware wise... You will most certainly benefit from upgrading your ram though, in every aspect of performance.